The Chemistry of Kissing
by JustlikeWater
Summary: Like most life-altering things, it began with a few simple words: "John, I need you to kiss me."


**A/N: I posted this on my other fan fiction account (siennna on AO3) last month and it got some pretty great results, so I thought I'd share it with you guys! Despite how cliché it is, I LOVE the 'it's for science john!' trope ^.^ Hope you guys like this, it was so much fun to write!**

**Enjoy!**

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><p>It was an uncharacteristically quiet day in 221B, and late-afternoon found John and Sherlock lounging in the sitting room with their pastimes of choice, as they reveled—or, in the latter's case, resented—the peaceful stillness of the evening. While Sherlock sulked on the couch, John unhurriedly typed up their latest case, a cup of tea steaming at his side. Time passed by with the fluidity of syrup, and their doorbell remained blessedly silent and client-free. In hindsight, John supposed that the tranquility of that afternoon should have been highly indicative of the strangeness that would come.<p>

John was on the third paragraph of "The Man with the Twisted Limp", when, with absolutely no preamble, Sherlock broke the standing four-hour silence.

"John, I need you to kiss me."

John's hand jerked in surprised, and he burned his lip on the hot tea he was in the midst of sipping. The question fluttered in the silence like a leaf on a wind current. Sherlock, laying supine on the couch with his eyes shut and his palms steepled against his lips, appeared as if he hadn't voiced the phrase at all. John peered at him incredulously.

"_Pardon?" _It wasn't so much a question as it was him giving Sherlock the benefit of the doubt. Surely John just misheard him…

Sherlock's eyes flew wide open as he exited his reverie and sat straight up, his dark curls disarray and wild from being smashed into the upholstery of the couch. He turned to face John with the same look of sharp focus he often wore during a complex case; his pale eyes were alight, his cupids-bow lips were pursed in concentration, and he was leaning forward with his elbows on his knees, eagerness seeping from every pore.

"John, you must. I was thinking about that case from two weeks ago, the one with the dead woman and that shady bloke; I could've solved it a whole day earlier if I'd had knowledge of the—apparently underrated—science of kissing. If I'd put it into consideration, I would've realized that _that_ was how the victims DNA ended up in that man's mouth, despite the lack of sexual contact. It would've seamlessly proved his alibi false. A simple exchange of saliva. So obvious, in hindsight."

John furrowed his brow. "You mean to say you've never kissed anyone?"

Sherlock's focused, almost business-like expression faltered, and his cheeks reddened. "Yes. I have never kissed anyone. If you haven't noticed, John, I'm not the most likable bloke."

John rolled his eyes and returned his attention to the screen. "Well, you've survived on theoretical knowledge thus far, I'm sure you'll be fine."

"Yes, John I believed the same until that bloody case. Whether I'd like to or not, I require practical knowledge on the subject." When John remained unmoved, Sherlock tried a different tactic; "So you're willing to risk the failure of another case, John? People could get hurt."

"Sherlock, I'm not bloody kissing you," said John with a finality he didn't quite feel. He made a point of reassuming his typing, loudly clicking the keys with two fingers.

"John."

"No, Sherlock."

"_John!"_ Sherlock groaned, dramatically flopping back down on the couch and throwing a forearm over his eyes.

Unmoved, John calmly replied, "You know a number of people, Sherlock. Find someone else. In fact, I'm sure Molly Hooper down at Bart's would be overjoyed to help with this little project of yours."

Sherlock removed his forearm and propped himself up on his elbows, his expression twisted in horror. "_Molly Hooper?_ John, how could you even _suggest_ such a thing?"

"What? She isn't bad; in fact, she's actually quite pretty, in a mousy, shy, sort of way."

Sherlock proceeded to roll his eyes so violently that John feared his irises might disappear into the back of his skull forever. "You don't _get_ it, John," he complained, "I don't _want_ to try this with someone else. The thought of merely brushing shoulders with anyone that isn't you is absolutely revolting. You know how I feel about _people_," he spoke the word with the same distaste one might use when discussing cockroaches. "If it's a matter of your sexual identity being compromised, then you hardly need to worry because no one but the two of us are here. Even if Mrs. Hudson pops in for some reason, she already thinks we're involved anyway. It'll just be a quick kiss, three minutes and thirty seconds ought to be sufficient, and then were done. Just think of this as any other experiment."

From Sherlock's mouth it sounded reasonable. "Two minutes."

"Two and a half," Sherlock retorted.

The mere fact that John was considering this seriously enough to begin bartering with the lunatic was probably a sign of his own impending madness. Then again, he'd participated in plenty of Sherlock's crazy experiments already, so why not? It wasn't as if he had much at stake here.

John took a deep breath and then released it in a semblance of defeat. "Fine."

Thankfully, the walk from John's chair to the couch did not last long enough for John to consider how absolutely bloody insane this whole thing was. If it had been, he probably wouldn't have seated himself next to Sherlock, scooted _way_ closer than usual, and turned to face him.

As it stood, the walk was short, John's inner voice hardly had time to mull anything over, and that was _exactly_ what happened.

"Okay, what are you going to do first?" Sherlock questioned, brow creased slightly in concentration, his eyes unsettlingly intent.

"Er, well, I suppose I'll put my hand on the side of your face." John scooted closer until their knees were touching and gingerly raised a palm to cup Sherlock's cheekbones. His skin was surprisingly warm, considering the seemingly-heatless pallor of his complexion.

"Why?" Sherlock asked, wide, absinthe-green eyes locked unerringly on John's.

"To…to guide you into the kiss, I suppose," John answered, his own voice coming out a bit strained. He cleared his throat. "Now I'm going to lean in and tilt my head a bit to the left and you'll tilt yours to the right and then—er, then we'll be kissing."

Sherlock blinked a few times then frowned. "How will our lips line up? Explain."

Christ, did the man want a diagram? Though, to be fair, this was Sherlock after all—the ever-inquisitive detective and purveyor of nit-picky questions—so it wasn't too surprising that he wanted to know every detail. John absently stroked his thumb over the soft plane of Sherlock's cheekbone and answered, "Well, usually person A kisses person B's top lip and person B kisses person A's bottom, or visa-versa. The point is, it allows both people's mouths to sort of—I don't know—slot together like puzzle pieces."

The detective considered this for a moment. "Which lip will you kiss?"

John's eyes fell to his mouth as soon as he spoke the question. Not that John had been _checking him out_ or anything, but in the back of his mind he'd always thought Sherlock's lip-shape was almost quite…pretty. Nearly feminine, actually. His top lip rose high at its peaks and then dipped low into the valley of his philtrum, giving him an appearance of aristocratic elegance. His bottom lip, on the other hand, John privately likened to lush, enticing fruit hanging illicitly from a branch. Aside from his brilliant eyes, Sherlock's mouth provided the only color on his entire face: just a smear of opulent red from a distance, and, up-close, a transfixing work of art.

Not that he was interested, of course. He was simply comfortable enough with his own sexuality to appreciate a fellow bloke's admirable features; it had absolutely nothing to do with the lurch he felt whenever the detective's bone-white teeth dug into the plush crimson of his fleshy bottom lip, nor did it have anything to do with his strange habit of fixating on Sherlock's mouth whenever he spoke.

John bit the inside of his cheek and crossed his legs, determinedly maintaining eye contact. "Your bottom lip," he answered steadily.

"Why?"

"It's…full and, er, easier to kiss." _Understatement of the bloody year._

Surprisingly, Sherlock accepted his answer and scooted closer, his own hand rising up to cradle the side of John's face. As he leaned in ever-so slowly, John noted that Sherlock's large palm was nearly the size of his face. _Christ._ In the dwindling moments before the kiss, several thoughts flashed through John's mind.

The first was a lightning-fast montage of ex-girlfriends in various sensual scenarios, each memory embossed with a neon sign reading "I am not gay". The next was Sherlock's mouth, as pink and delectable as a juicy peach, and the subsequent thought of "fuck it". His final notion, however, was cut short, because as soon as his lips met with Sherlock's, all internal contemplation ceased.

Just as he'd suspected, Sherlock's mouth was warm and pliant beneath his, tentatively moving against his own in an unsure manner that was quite uncharacteristic for a man who typically oozed confidence. His fingers squirmed nervously against the side of John's face, as if to ask 'is this okay? Am I doing this correctly?' John used his free hand to stop Sherlock's moving fingers and eagerly latched on to the plump curve of his bottom lip, sucking gently. John made a soft noise in the back of his throat that he'd intended as reassuring confirmation, but ended up sounding more like a moan. His face reddened and he expected Sherlock to pull back in distaste—after all, Sherlock was doing this for science rather than pleasure. However. Sherlock's reaction was quite different. Instead of pulling away and leaving, Sherlock stilled for a moment in shock, then, in a flurry of motion, tangled his fingers in John's hair and pulled him closer with a growl, the kiss quickly transitioning from chaste and slow, to a wet mess of tongues, lips, and teeth.

John was more than happy to reciprocate, but found that his surprise—and more importantly, his confusion—were too vast to suppress. "Wha—_what?"_ he panted, pulling their mouths apart with an obscene-sounding, wet pop. Sherlock ignored his question and tried to reattach their lips, then when John refused, dove to his neck instead, sucking messy open-mouthed kisses up the side of John's throat. John gasped as if he'd been electrocuted and thoughtlessly tangled his fingers in Sherlock's curls, holding the detective's bowed head against his collarbones so he could continue doing those delicious things with his tongue. He was clearly inexperienced, a bit too imprecise and sloppy, but what he lacked in skill he made up for in eagerness and his innate ability to pick things up remarkably fast. "Do it again," he panted against John's neck, sucking yet another love-bite into existence. "Make that noise again."

What noise? Did he mean—_oh._ "That's what's got you all riled up?" he chuckled breathlessly, running his hands fervently up and down Sherlock's curved back as he licked a stripe over John's Adams apple. "Just a little—ah—moan?"

'"It's a good sound," Sherlock retorted, alternatively nipping and lapping at John's pulse point.

"Right, well—" and then whatever smart-arse comment John had lined up immediately dissolved, in the place of an embarrassingly loud, rumbling moan. He could practically feel Sherlock smirking against his skin as he kissed lazy patterns up the side of John's neck. "_Jesusfuckingchrist"_ John panted, knotting his fingers into Sherlock's dark curls. "How the hell are you doing that?"

Sherlock sucked John's earlobe teasingly between his teeth, one hand gripping John's shoulder and the other tugging idly through his hair. "With my mouth," he responded drily, clearly amused. John was well aware that Sherlock was mocking him, but it was hard to take offense when his voice was so goddamn _hot_—John could practically feel that rich, deep baritone rolling over his skin like fingertips. It was bloody _wonderful._

After sucking a sufficient number of love bites across John's throat, Sherlock returned his attention to John's lips, pecking the corner of his mouth and his cheek teasingly, purposefully avoiding a full kiss. "Nope, none of that," John growled, placing both hands on the sides of Sherlock's face and tilting his head so that—_holy mother of god. _With their mouths connecting at a more extreme angle, John found that he could easily lick his way into Sherlock's mouth, his tongue curling delightfully against Sherlock's, and it felt bloody amazing. If the growly moan in Sherlock's throat was anything to go by, he was clearly enjoying it just as much as John.

"J-John, you're," Sherlock panted, sucking shamelessly at John's bottom lip, "—_perfect."_

John grinned, breathless, and continued worshipping Sherlock's mouth, his right hand scraping pleasurably through the detective's wild curls. "I could—ah—say the same about you." He peppered those gorgeous cheekbones with eager kisses, unhurriedly making his way back to Sherlock's ear. "Hey, do you wanna maybe take this elsewhere?" _Like a bedroom, for example?_

But, because the universe clearly had a vendetta against John and desperately didn't want him to get laid, they were interrupted. "Boys, care for some chocolate biscuits?" Mrs. Hudson called from downstairs. "They're fresh out of the oven!"

Sherlock pulled back and blinked at him, his eyes cloudy and his lips looking swollen and thoroughly snogged. Without looking away, he called, "No, thank you, Mrs. Hudson."

"Alright, boys. If you change your mind, you know where to find me!"

John untangled his limbs from Sherlock's—wow, since when had his hand crawled up the back of Sherlock's shirt?—and folded his hands in his lap, defeated.

Suffice to say, the mood was killed.

"So, er, have you gained sufficient knowledge?" John asked, flustered and still breathing hard.

Sherlock affected disappointment and sighed. "Unfortunately, the results were inconclusive. I believe we will have to revisit this experiment. Repeatedly."

John coughed, cheeks growing warm. "Right, yes, of course. Just to be sure."

Sherlock nodded. "Yes, accuracy is very important to me, John." He paused, considering, then said, "You know, in an experiment, it's always best to have an independent and dependent variable. In the future, there will be many occasions with kissing, so that will be our dependent variable. The independent variable that I feel we should test next, however, is kissing with—"

"Less clothes," John finished.

Sherlock smirked, his eyes bright and mischievous. "Why, Doctor, you read my mind."

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><p><strong>AN: So, what did you think? Hate it, love it? Lemme know in the comments, guys! **

**Hoped you enjoyed this little one shot, feedback would be gloriously appreciated XOXO**


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